Numerous industrial processes discharge streams which comprise a mixture of various gaseous fluids. There is increasing concern that some of the constituents of the effluent streams may cause significant environmental problems, and that these streams therefore should not be released into the atmosphere. Carbon dioxide is a compound which is a constituent of many of the effluent streams released from industrial processes and whose release into the atmosphere is causing increasing concern.
It is hypothesized that carbon dioxide released into the atmosphere acts as a green-house gas and that too high a concentration of green-house gases in the atmosphere will cause global warming. In response to this potential threat, many governmental bodies have either enacted or plan to enact regulations limiting the quantity of carbon dioxide which can be released into the atmosphere. These regulations can hamper many industries because the combustion of virtually any hydrocarbon fuel with air produces an effluent containing carbon dioxide, nitrogen, and other gaseous combustion products.
The mixture of gases which results from the combustion of a hydrocarbon with oxygen or air is hereinafter referred to as "flue gas." The chemical composition of flue gas depends on many variables, including but not limited to, the combusted hydrocarbon, the combustion process oxygen-to-fuel ratio, and the combustion temperature. In addition to carbon dioxide and nitrogen, flue gas may contain compounds such as hydrogen sulfide, carbon monoxide, sulfur oxides, nitrogen oxides, and other constituents. The release of these compounds to the atmosphere also is coming under increasing public scrutiny and is the subject of increasing governmental regulation.
In addition to being a hydrocarbon combustion product, carbon dioxide can be produced by natural processes and released to the environment during a non-combustion process. For example, carbon dioxide is produced by the thermal and biogenic processes which are believed to form hydrocarbons such as oil, natural gas, or coal. Carbon dioxide often is recovered with these hydrocarbons and released to the atmosphere by various post-production steps. An example of such a process is cryogenic separation, which utilizes cryogenic cooling and compression to separate non-hydrocarbon fluids, such as carbon dioxide, from a natural gas production stream.
The increasing concern over the atmospheric release of carbon dioxide and other compounds demands methods by which to dispose of the compounds. Because the waste compounds often are a constituent of a volumetrically larger effluent stream, it is preferred that the disposal methods somehow utilize the larger effluent streams to enhance the efficiency of the overall process and/or to facilitate the recovery of a valuable product using the process in addition to providing for the disposal of carbon dioxide contained therein.
As used herein, the following terms shall have the following meanings:
(a) "cleats" or "cleat system" is the natural system of fractures within a solid carbonaceous subterranean formation; PA1 (b) a "coalbed" comprises one or more coal seams in fluid communication with each other; PA1 (c) "desorbing fluid" includes any fluid or mixture of fluids which is capable of causing methane to desorb from a solid carbonaceous subterranean formation; PA1 (d) "formation parting pressure" and "parting pressure" mean the pressure needed to open a formation and propagate an induced fracture through the formation; PA1 (e) "fracture half-length" is the distance, measured along the fracture, from the wellbore to the fracture tip; PA1 (f) "recovering" means a controlled collection and/or disposition of a fluid, such as storing the fluid in a tank or distributing the fluid through a pipeline. "Recovering" specifically excludes venting the fluid into the atmosphere; PA1 (g) "reservoir pressure" means the pressure of a productive formation near a well during shut-in of that well. The reservoir pressure can vary throughout the formation. Also, the reservoir pressure of the formation may change over time as desorbing fluid is injected into the formation and fluids are produced from the formation; PA1 (h) "solid carbonaceous subterranean formation" refers to any substantially solid carbonaceous, methane-containing material located below the surface of the earth. It is believed that these methane-containing materials are produced by the thermal and biogenic degradation of organic matter. Solid carbonaceous subterranean formations include but are not limited to coalbeds and other carbonaceous formations such as antrium, carbonaceous, and devonian shales; PA1 (i) "well spacing" or "spacing" is the straight-line distance between the individual wellbores of a production well and an injection well. The distance is measured from where the wellbores intercept the formation of interest; PA1 (j) "preferentially sorbing", "preferentially sorbs", and "preferential sorption" refer to processes which occur within a solid carbonaeeous subterranean formation that alter the relative proportions of the components of a gaseous fluid. These processes may fractionate a mixture of gaseous fluids by equilibrium separation, kinetic separation, steric separation, and/or any other physical or chemical processes or combination of processes which within a solid carbonaceous subterranean formation will selectively fractionate a mixture of gaseous fluids; PA1 (k) "raffinate" refers to that portion of a mixture of gaseous fluids injected into a solid carbonaceous subterranean formation which is not preferentially sorbed by the formation; and PA1 (1) "adsorbate" is that portion of a mixture of gaseous fluids which is preferentially adsorbed by a carbonaceous matrix of the solid carbonaceous subterranean formation and which is recovered from the formation when the total pressure within the formation is reduced. PA1 a) introducing a mixture of gaseous fluids comprising a weaker adsorbing fluid component and a stronger adsorbing fluid component into the coal seam: and PA1 b) recovering a raffinate, enriched in the weaker adsorbing fluid component, from the coal seam. In a second aspect of the invention, a method is disclosed for recovering methane from a solid carbonaceous subterranean formation penetrated by an injection well and a production well, the method comprising the steps of: PA1 a) injecting a desorbing fluid having a volume percentage of carbon dioxide equal to A into the solid carbonaceous subterranean formation through the injection well; PA1 b) recovering an effluent comprising methane from the production well; PA1 c) monitoring the composition of the effluent produced in step b); and PA1 d) ceasing recovery of the effluent produced in step b) when a volume percentage of carbon dioxide in the effluent recovered in step b) is greater than 0.5 A. PA1 a) injecting a desorbing fluid, having a volume ratio of carbon dioxide to other injected desorbing fluid components equal to B, into the solid carbonaceous subterranean formation through the injection well; PA1 b) recovering an effluent comprising injected desorbing fluid and methane from the production well; PA1 c) monitoring the volume ratio of the carbon dioxide to other injected desorbing fluid components contained in the effluent recovered at the production well; and PA1 d) ceasing recovery of the effluent from the production well when the volume ratio of carbon dioxide to other injected desorbing fluid components within the effluent recovered at the production well is greater than 0.5 B and at least 70 percent of the methane available to the production well has been recovered. PA1 a) injecting a desorbing fluid, having a volume ratio of carbon dioxide to other injected desorbing fluid components equal to B, into the solid carbonaceous subterranean formation; PA1 b) withdrawing a gaseous effluent having a volume ratio of carbon dioxide to other desorbing fluids of less than B from the formation; and PA1 c) ceasing to withdraw the gaseous effluent from the formation when a volume ratio of carbon dioxide to other injected desorbing fluid components within the gaseous effluent withdrawn in step b) is greater than 0.5 B. PA1 a) injecting a desorbing fluid having a volume percentage of carbon dioxide equal to A into the solid carbonaceous subterranean formation through the injection well; and PA1 b) recovering an effluent having a volume percentage of carbon dioxide less than A from the production well.